Certain alumina fiber based heat insulating materials are well known. Such materials have a composition resulting from the mixing of alumina fibers and of silica fibers, or from the mixing of three types of fibers: alumina, aluminoborosilicate and silica. These materials are manufactured in a V-blender from an aqueous suspension of fibers, with subsequent drying and firing. Materials of this type cannot be used for a long period of time at a temperature in the region of 1600.degree. C. because silica fibers which are not sufficiently heat resistant are present in the composition. In addition, the composition contains boron oxide, which is introduced therein to obtain a better sintering and to prevent the formation of .alpha.-cristobalite. The maximum temperature, for a prolonged operating or service life, is 1430.degree. C. in the case of these materials.
European patent application No. 0 093 656 discloses a heat insulating material based on fibers of alumina, carbon and other refractory materials. Different organic resins are used for binding the fibers. A molded fibrous billet is impregnated with the resin and is subjected to pyrolysis in an inert atmosphere at a temperature of 700.degree. to 2700.degree. C. This material also cannot be used at high temperature because the binder is a product of the pyrolsis of organic resins. Consequently, the material has a tendency to oxidize in air, at temperatures of 900.degree. to 1100.degree. C. As indicated in the aforementioned patent application, the resulting material is capable of withstanding only 1000.degree. C. for 12 hours and a thermal shock of up to 3000.degree. C. This material has a density of 200 kg/m.sup.3 (0.2 g/cm.sup.3) and a bending strength of 4 MPa (40 kgf/cm.sup.2).
A strong, lightweight heat insulating material is described in French patent application No. 2 669 623. The bulky parts of intricate configurations are made by weaving the skeleton from carbon fibers pre-treated with a first binder, and then this skeleton is rigidified by means of a second binder and subjected to heat treatment. The heat resistance of this material is the same as for the material described in the aforementioned European patent application. As the binders used in the invention described are carbon precursors, the service temperature cannot exceed 1000.degree. C., for the same reasons as given above.
Ceramic and/or alumina fiber based materials having densities in the range of 144 to 1100 kg/m.sup.3 (0.144 to 1.1 g/cm.sup.3) are well known from U.S. Pat. Nos. 4,041,199, 4,349,637 and 5,071,798; German patent applications 27 00374, 33 15880, 34 44397 and 38 05110; British patents 1 433 923 and 1 506 152; British patent applications 2 052 472 and 2 089 782; and European patent applications 0 010 385 and 0 371 586. These materials are proposed for use as high temperature insulation (in general as furnace linings) at temperatures ranging from 1400.degree. to 1800.degree. C. In practice, the composition of each of the above materials includes refractory fibers or materials (alumina, silica powder or a mixture of oxide powders), refractory binder (colloidal alumina and/or colloidal silica, or precursors of these oxides, or their mixtures and more complex compositions), thickeners, stabilizers, activators and other processing aids.
Processes for manufacturing articles (plates and more complex parts) differ from one another but include common steps: molding an aqueous suspension of fibers, introducing a binder, subsequent drying and firing of the fibrous billet obtained. The binder is introduced either directly into the suspension, or by impregnating the green fibrous billet, or both processes are used if the binder is complex and if it is desirable to introduce the ingredients of the binder in succession.
Molding is carried out using a vacuum process, under pressure, by casting or using a combination of different processes, for example casting with simultaneous vibrations, etc.
The resulting material has a ratio of fibers to binder (or to other fillers) extending over a very wide range. The ratio of the fibers withstanding the highest temperatures ["Saffil.RTM." made by the LOI company (Great Britain), "Fibermax.RTM." made by the Carborundum Resistant Materials company (USA), "Denka Alken.RTM." made by Denki Kagaku Kogyo K.K. (Japan)] to the less heat resistant ceramic fibers ["Fiberfrax.RTM." made by Carborundum Resistant materials (USA), "Triton Kaowool.RTM." made by Marganit Ceramik Fiber (Great Britain), etc. . .] also extends over a very wide range. It is obvious that the higher the "high temperature" fiber content of the material, the higher is its heat resistance.
The highly heat resistant fibers "Saffil.RTM.", "Fibermax.RTM." and "Denka Alken.RTM." have a mean diameter of 3 .mu.m, a strength of 500 to 1000 MPa (50 to 100 kgf/mm.sup.2), a maximum operating temperature of 1500.degree. to 1700.degree. C., that is to say a classification temperature of 1500.degree. to 1700.degree. C., and the following composition (in weight %):
______________________________________ "Saffil .RTM.": 96 Al.sub.2 O.sub.3, 4 SiO.sub.2 ; "Denka Alken .RTM.": 80 Al.sub.2 O.sub.3, 20 SiO.sub.2 ; "Fibermax .RTM.": 72 Al.sub.2 O.sub.3, 28 SiO.sub.2 (mullite ______________________________________ composition).
In U.S. Pat. No. 5,071,798 and in German patent application (DE-OS)38 05110, there is disclosed a material having a service temperature of 1600.degree. to 1800.degree. C. This material is formed by placing in an aqueous suspension a mixture comprising highly heat resistant Al.sub.2 O.sub.3 fibers having a mean diameter of 3 .mu.m and aluminoborosilicate fibers, Al.sub.2 O.sub.3 and/or SiO.sub.2 powder, a binder constituted by colloidal Al.sub.2 O.sub.3 and/or SiO.sub.2 and aluminum sulfate. The proportions of the components are selected such that, after firing, which is carried out at a temperature of 1400.degree. to 1600.degree. C., the composition consists solely of mullite (over 12 weight %) and of corundum. For different reasons, for example owing to the placing of the powder in an aqueous suspension without any thickener, the process which is disclosed in the aforementioned documents does not make it possible to obtain a uniform material having a density of less than 500 kg/m.sup.3 (0.5 g/cm.sup.3), the reason for this being the precipitation of the powder on the lower surface of the plate under the effect of gravity. In addition, the need to carry out the firing step at high temperatures leads to grain growth and, consequently, the strength of the fiber decreases. It appears that, even when the material is of rather high density, [810 kg/m.sup.3 (0.81 g/cm.sup.3)], its maximum bending strength is only 10.2 MPa (102 kgf/cm.sup.2).
There is known a material having a bending strength of 0.3 MPa (3 kgf/cm.sup.2), whereas its density is 250 kg/m.sup.3 (0.25 g/cm.sup.3) (see British patent 2 089 782). This material is intended for a service temperature of 1600.degree. C. and exhibits 3% linear shrinkage after 3 hours at 1600.degree. C.
As a thickener, use is made of carboxymethylcellulose or polyethylene oxide which also acts as a dispersant to produce a uniform slurry.
Cation starch is widely used as a thickener (see GB patent 1 506 152 and German patent applications 27 00374 and 34 44397).
The ceramic fiber based material having thermal resistance of up to 1260.degree. C. (i.e. fibers that are well sintered at low temperatures) has maximum strength. The thermal resistance of these fibers thus also defines the thermal resistance of the material.
When "high temperature" fibers are used, it is necessary to provide more thermally resistant binder which, in turn, leads to higher temperature sintering. This sintering temperature is sometimes higher than the service temperature of the material, but, in such a case, the strength of the fiber is reduced because of the grain growth.
Increasing the density of the material is the alternative way of obtaining a strong material.
To obtain a strong, lightweight composition, researchers use different methods of introducing the binder. In addition, binder composition vary over a wide range for the purpose of obtaining a sintering temperature that is at least not higher than the service temperature of the material.
As disclosed in U.S. Pat. No. 4,041,199, colloidal silica (SiO.sub.2) is introduced into an aqueous powder-fiber mixture, in parallel with starch. In addition, aluminum powder and aluminum sulfate are also added (in the amount of 1 weight %, based on the total weight of all the components in the resulting material), in order to reduce the density of the product.
GB patent 2 052 472 discloses a process that comprises wetting a fibrous mat with an aqueous monoaluminum phosphate solution.
German patent application (DE-OS) 33 15880 teaches a process according to which an aluminum hydroxide gel and/or an aluminum salt solution are placed in an aqueous fiber slurry, at the same time as a solution of monoaluminum phosphate. The manufactured article is obtained by vacuum molding with additional drying.
A refractory binder composition is suggested in aforementioned U.S. Pat. No. 4,349,637. The binder includes alumina (Al.sub.2 O.sub.3) or other complex compounds containing aluminum which are transformed into Al.sub.2 O.sub.3 during drying and firing.
Above-mentioned European patent application 0 371 586 discloses the introduction of an organic binder (polyvinyl alcohol, polyethylene oxide, etc.) into a non-organic binder (colloidal Al.sub.2 O.sub.3, SiO.sub.2, ZrO.sub.2, and their mixtures).
From aforementioned GB patent 1 433 923 is known a binder comprising a sol formed of colloidal particles having a dense silica core and a colloidal amorphous alumina coating.
Although the aforementioned processes are advantageous, "high temperature" fiber based lightweight materials [having a density of 100 to 500 kg/m.sup.3 (0.1 to 0.5 g/cm.sup.3)], which materials are acceptable for long term service at 1600.degree. C., have low strength.
Thus, the maximum bending strength of the material having a density of 250 kg/m.sup.3 (0.25 g/cm.sup.3) is 0.3 MPa (3 kgf/cm.sup.2) (see GB patent application 2 089 782) and the maximum bending strength of the material having a density of 520 kg/m.sup.3 (0.52 g/cm.sup.3) is 4.2 MPa (42 kgf/cm.sup.2) [see German patent application (DE-OS) 34 44397].